The lenses of the type used in cameras of the 35 millimeter film size in single lens reflex cameras have had to have long back focal lengths in order to provide room for a movable viewing mirror. Zoom lenses for cinematography and video applications have not been restricted to a long back focal length and usually have relative apertures in excess of f/2.8 which must be held during zooming. For these reasons, it has not been customary to utilize the lens unit nearest the image plane for the primary magnification change, that is, the variator function.
Recently, zoom lenses have been introduced for 35 millimeter cameras of the type which do not utilize a mirror for through-the-lens viewing. An example is shown in Japanese patent application laid open No. 57-201213. In this disclosure, a first positive lens unit is followed by a rear negative lens unit, both lens units moving to change the equivalent focal length (EFL) of the lens. The first lens unit is of a modified triplet type construction having a strong central negative element, while the rear lens unit consists of a positive meniscus element followed by a negative meniscus element. This design is claimed to have a focal range of less than two to one. However, it would not be difficult to extend the range to about two to one by compounding the elements and through the efficient use of aspheric surfaces. These lenses, at the present time, may be found in viewfinder type 35 millimeter cameras.
Lenses of this type are also disclosed in U.S. Pat. Nos. 4,659,186; 4,679,913; 4,682,860; 4,720,179; and 4,721,371, all of which utilize a triplet or modified triplet front group.
The type of design discussed above is capable of a very short front vertex to image length at the storage position. Typically, it is about one and one-half times the minimum EFL, depending upon the EFL range. As a result, this lens type is successful for compact cameras but a two to one EFL range limits its usefulness.
Extending the EFL beyond a range of two to one has not been accomplished because of a fundamental limitation of the modified triplet construction. The basic triplet form of lens is well known to have adequate degrees of design freedom to correct all the primary aberrations. This has been the reason for the popularity and success of this design type for many years. But these features of the triplet type lens do not make triplet type lenses ideal for applications in zoom lenses where it is not desirable to correct all the primary aberrations in any particular lens unit.
In the triplet design, the central negative element normally will have considerable amounts of overcorrected spherical aberration. To correct this is very difficult, requiring considerable complexity, due to the somewhat conflicting distortion and astigmatism introduced at the short EFL (wide angle) position. Similarly, the triplet lens form is suitable for correction of Petzval curvature. The negative element of the triplet more than compensates for that of the positive elements if normal glasses are used. It is thus apparent that neither lens unit is individually corrected, and the triplet is used in an abnormal manner.
A second disadvantage of the triplet type of first lens unit is that the triplet usually has a principal point near the center of the lens or, in some applications, near the object side. For a compact zoom lens with a first positive lens unit and a second negative lens unit, this means the amount of zooming space is reduced by the distance from the last lens vertex to the second principal point. This results in the use of greater optical powers, and thus, more severe requirements for aberration correction for a given aperture and field angle.
Accordingly, the present invention provides a new and improved zoom lens of the type described of simplified construction and extended zoom range.